1. Field of the Invention
This invention relates to a process for laminating two sheets wherein one has a pressure-sensitive surface and more particularly it relates to a process whereby the pressure required for contacting the two sheets is applied from the back side of the laminate.
2. Description of the Prior Art
Tufted carpet is made conventionally by use of specially designed machines which tuft loops of yarn into a suitable backing or "primary scrim" material such as woven jute, woven or spunbonded synthetic resins, etc. The loops may be left intact, or some or all may be cut. Also, the loops may be equal in size or may be at two or three different heights, depending upon the final desired effect. The tufted carpet is then usually dyed, by means of a "beck" or, alternatively, by continuous dyeing or printing of a design, or both, again depending upon the desired aesthetic effect. At this stage, while the unfinished carpet ("soft carpet") is aesthetically pleasing, it is not stable as individual tufts or loops are not firmly anchored to the primary scrim and can be removed by a very small force. Thus, it is imperative to use an adhesive to bond the structure. A commonly used method requires application of an adhesive to the back side of the "soft carpet," generally by use of a pan of adhesive plus a transfer roll, plus adding a secondary web ("secondary scrim") to the freshly applied adhesive on the primary scrim. The secondary scrim is customarily a woven sheet of lightweight jute, although use of other materials, such as woven or spunbonded polypropylene webs, is gaining in popularity. The final structure is now heavier, stiffer, and stronger, and will withstand many years of hard service without loss of tufts of face yarn, which are firmly held by the adhesive between two strong webs. Also, the bottom of the rows of stitches has been covered, which improves the appearance of the back side of the carpet.
When any two structures are glued together, the adhesive must "wet" both of the parts to be mated if a strong bond is to be formed. If one of the faces to be adhered is not well contacted by the adhesive, the final adhesive bond will be very weak. In laminating textile or other webs, it is common to use a set of squeeze or "nip" rolls which "marry" the final "sandwich" of primary scrim-adhesive-secondary scrim as shown in U.S. Pat. Nos. 3,551,231 and 3,684,600. The tackiness of the adhesive holds the structure together until the adhesive has set (dried, cured, or hardened) to form a strong bond between the primary and secondary webs.
Some of the adhesives used today are based upon an aqueous latex (emulsion or dispersion), which is cured by water removal plus, in some instances, chemical reaction. Plastisols or organosols (organic solvent based adhesives) may also be used. However, new adhesive types -- the hot melts -- are gaining in favor because these set rapidly as the melt congeals, do not require curing/drying ovens, operate at high line speeds, and thus conserve both utilities and labor. In all cases, whether a hot melt or a cold adhesive is used, it is imperative to secure intimate contact between the adhesive and both the primary and the secondary webs. As the adhesive is usually applied directly to the primary web, contact with it is excellent. But, it is also essential to be sure the secondary web is also well wetted. This is generally accomplished by adding the secondary web to the still wet and tacky primary web and then pressing the two webs firmly together. In a continuous process, such as in the lamination of textiles or, in particular, in the backcoating of carpets, the needed pressure is secured by use of one or more sets of nip rolls. Of necessity, this action transmits substantial pressure to both the face side and the back side of a carpet.
When the face side of the carpet passes under a nip roll, the pile of the carpet is crushed. In the case of a loop-pile carpet or a short plush carpet, the deformation is small and not readily noticed. However, the amount of pile distortion (pile forced backwards away from the flow of carpet) for a medium-cost or low-cost long shag carpet is appreciable and might cause an adverse reaction or a rejection by a prospective purchaser.
When pile distortion due to action of the top nip roll or other object which forces the pile backwards occurs, it can be corrected by means such as brushing. However, this is an attempt to correct an earlier mistake -- and also introduces new problems. Brushed carpet has inferior tuft definition (i.e., individual tufts become fuzzy and seem to merge), and the fuzz created by the brush soon will plug up the brush, requiring a shutdown plus a slow cleanup operation. Other corrective means, such as air jets, air knives, heating, steaming, etc. can also be employed to correct pile lay. These too have drawbacks, such as pile disturbance, equipment cost, moistening of carpet, high utilities needs, etc. Thus, it is better to avoid the pile distortion problem entirely than to try to correct the problem later.
To eliminate this problem U.S. Pat. No. 2,675,337 (Walker et al.) suggested means to avoid pile flattening by "uniting the fabrics under a pressure that is applied by drawing them both together under tension round a curved surface" (Col. 3, Lines 19-21). However, as indicated in Col. 3, lines 64-70, part of the time the web is under substantial forces applied by rollers 19 and 23 (FIG. 3). With a pressure-sensitive surface and a partly-adhered backing, rollers 19 and 23 would cause pile lay in carpets. Rollers 19 and 23 are essential for Walker's system, if they were removed from his apparatus, his system would become inoperative. Thus, it is apparent that Walker did not recognize the need to avoid all pressure and frictional drag once the adhesive was applied.